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From the Istituto di Ricerche Farmacologiche Mario Negri, Department of Vascular Medicine and Pharmacology, "A. Valenti" Laboratory of Thrombosis Pharmacology, Consorzio Mario Negri Sud, Santa Maria Imbaro, Italy (F.Z., A. Di C., C.A., A.D, M.B.D., L.I.); and Gaubius Laboratory, Leiden, The Netherlands (L.I.).
Correspondence to Licia Iacoviello, MD, Department of Vascular Medicine and Pharmacology, "A. Valenti" Laboratory of Thrombosis Pharmacology, Consorzio Mario Negri Sud, 66030 Santa Maria Imbaro, Italy. E-mail iaco{at}cmns.mnegri.it
| Abstract |
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Key Words: fibrinogen polymorphisms myocardial infarction family history
| Introduction |
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, B-ß, and
-chains, arranged symmetrically. The three genes
encoding the three chains are located in a cluster of about 50 kb on
the long arm of chromosome 4.19 20 Humphries et
al15 21 demonstrated an association between the
Bcl I polymorphism of the ß-chain gene of fibrinogen
and fibrinogen levels. In the latter study on 91 English subjects, the
B1B1 homozygotes had a mean fibrinogen concentration of 274 mg/dL, the
heterozygotes had a concentration of 298 mg/dL, and the homozygotes for
the B2 allele had a mean concentration of 369 mg/dL. In a similar
study, Berg and Kierulf16 were not able to
confirm these findings in Norwegian subjects. The possibility that the ß-chain fibrinogen genotype may have an effect on the risk for arterial diseases has also been investigated. In the Edinburgh Artery Study, there was a higher frequency of the B2 allele of the ß-chain fibrinogen gene in patients with peripheral artery disease than in control subjects.9 However, there was no correlation found between the polymorphism and blood fibrinogen levels either in patients or in control subjects. More recently, Behague et al17 studied the impact of several fibrinogen ß-chain polymorphisms on the outcome of coronary artery disease. They found that only the B2 allele of the Bcl I polymorphism was associated with the severity of coronary stenosis, but not with the occurrence of acute myocardial infarction (AMI), suggesting an interaction between this genotype and the development of atherosclerotic complications. ß-Chain formation is the rate-limiting step in the assembly of the molecule, and its genetic modification could be responsible for changes in synthesis and activity of the fibrinogen molecule.22 All together, these data suggest that some genetic variability near the Bcl I ß-chain locus may be involved in the pathogenesis of cardiovascular ischemic disease, although such involvement has never been determined for AMI. The purpose of this study was to investigate the association of the Bcl I ß-chain fibrinogen polymorphism with the risk of AMI and its relationship with fibrinogen levels in the Italian population. We studied a homogeneous sample of Italian AMI patients with a high likelihood of inherited risk, defined by the presence of a family history of thrombosis.
| Methods |
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One hundred seventy-three control subjects without AMI, stable or unstable angina, stroke, or transient ischemic attacks, were consecutively selected among subjects attending the hospitals for any clinical reason except acute conditions.25 Subjects reporting personal or family history of thrombosis (AMI, stable or unstable angina, stroke, or transient ischemic attack), with defined defects of the hemostatic system, and with chronic liver diseases were excluded. Data were collected by ad hoc trained interviewers, using a structured questionnaire that included personal data, cigarette smoking, and medical history (diabetes, hypertension, hyperlipidemia). Diabetes was considered to be present if the patient was under treatment or considered by the admitting physician to be diabetic. Hypertension and hyperlipidemia were considered only if the patient was under antihypertensive or hypolipemic treatment. All interviewers were trained and checked for reliability and consistency.24 The subjects included in this study were all Italian and were distributed throughout the main Italian geographic areas (north 31%, center 19%, south 42%, and Sardinia 8%, for both cases and control subjects).
This work was performed according to the Declaration of Helsinki of 1975 and was approved by the Mario Negri Sud Ethical Committee.
Blood Samples
Patients were interviewed within 5 to 7 months of their most
recent ischemic event. Blood sample collection was performed
between 8 and 10 AM, after 20 minutes' supine rest, from
subjects who had fasted overnight and had refrained from smoking for at
least 6 hours before blood sampling. Patients under oral anticoagulant
treatment were excluded.
Venous blood was collected from an antecubital vein without stasis into plastic syringes, added to 3.8% sodium citrate (9:1, vol/vol) in precooled plastic tubes, and kept on ice until centrifugation. Plasma was obtained by centrifugation at 2000g for 20 minutes at 4°C, and aliquots were frozen at -80°C until testing.
Plasma fibrinogen concentrations were assayed by the modified Clauss functional method (Dade, Miami, Fla; MLA 1600; interassay and intra-assay coefficients of variation being 4.7% and 2.9%, respectively).
DNA Extraction and Bcl I Polymorphism
Detection
Peripheral venous blood samples were drawn, and
white blood cells were separated. DNA was extracted from
peripheral blood using standard procedures. Amplification
of the ß-chain fibrinogen gene was obtained by polymerase chain
reaction followed by gel electrophoresis. Fifty microliters of
polymerase chain reaction contained 100 ng genomic DNA, 200 ng of each
appropriate primer, 10 mmol/L Tris/HCl, pH 8.3, 1.5 mmol/L
MgCl2, 50 mmol/L KCl, 0.01% (wt/vol)
gelatin, 0.1% Triton X-100, 200 mmol/L dNTPs, and 1 U of
Taq polymerase (Promega Corporation). Samples were incubated
at 95°C for 5 minutes, followed by 30 cycles of 95°C for 1 minute,
55°C for 1 minute, and 72°C for 1 minute. The primers used were
(5'-3') ACC TGG TTT CTC TGC CAC AAG (coding strand) and AAT AGT TCT CAT
ACC ACA GTG T (noncoding strand).26 Ten
microliters of the polymerase chain reaction product was digested
with 10 U of the Bcl I restriction enzyme (Promega
Corporation) and run by electrophoresis in a 1.5% agarose gel and
visualized directly by ethidium bromide staining. Two alleles, B1
and B2, were detected at 2500 bp and 1100+1400 bp, respectively.
Statistical Analysis
Data were analyzed by the Mario Negri Sud mainframe
computer with the SAS statistical package. The frequencies of the
alleles and genotypes among cases and control subjects were
counted and compared by the
2 test with the
values predicted by assumption of the Hardy-Weinberg equilibrium.
2 analysis or Fisher's exact test was
used to compare differences between discrete parameters.
The differences between cases and control subjects were
analyzed by unpaired Student's t test for
fibrinogen levels and by the Kruskal-Wallis test for age, according to
their observed distribution. Odds ratios (ORs) as estimators of
relative risk, together with their 95% approximate confidence
intervals (CIs) were computed to assess the association with disease of
(B1B2+B2B2) genotype in relation to the B1B1 genotype.
Multiple logistic analysis was performed by using LOGISTIC
procedure for SAS; confounding variables included were age, gender,
smoking habits, history of hyperlipidemia,
hypertension, and diabetes. In a separate analysis, the
genotype effect on AMI risk was also adjusted for fibrinogen
levels. A general linear model (unbalanced ANOVA for two-way design
with interaction) was used to assess differences of fibrinogen levels
in patients versus control subjects, in B1B1 versus (B1B2+B2B2)
individuals, and to assess the interaction between case status and
Bcl I genotype. Fibrinogen levels were adjusted for
gender and smoking habits.
All the results are given as mean±SD. A value of P<.05 was considered significant.
| Results |
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Genotype distributions and B1 and B2 allele frequencies at
the Bcl I ß-fibrinogen locus are shown in Table 1
. Genotype distribution was in
Hardy-Weinberg equilibrium in the control group, whereas it deviated
from equilibrium (
2=4.3, df=1,
P=.04) in the group of AMI patients. The frequency of
allele distribution was significantly different in patients versus
control subjects (P=.002), the B2 allele frequencies
being, respectively, 0.28 versus 0.17. Genotypes were
differently distributed between cases and control subjects
(P=.002, Fisher's exact test). Because of the low number of
B2B2 homozygotes, we considered individuals carrying the B1B2 and B2B2
genotype as only one group (B1B2+B2B2) for further
analyses. The genotype frequencies were still largely
different between cases and control subjects (P<.001):
there was an excess of subjects carrying the B2 allele in AMI
patients compared with control subjects (53% versus 32%).
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The characteristics of the two groups and the relative ORs are shown in
Table 2
. Cases were slightly older than
control subjects and had a higher prevalence of common risk factors for
atherosclerotic disease (such as smoking habits, history of diabetes,
hyperlipidemia, and hypertension). After
multivariate analysis, only age, smoking, and
hyperlipidemia remained significantly associated with
the risk of familial AMI.
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Patients also showed plasma fibrinogen levels higher than control subjects (310±76 versus 243±50 mg/dL, P<.0001). The OR attributed to the increase of 10 mg/dL in fibrinogen levels was 1.23 (95% CI 1.16 to 1.30) in univariate analysis and was not modified by adjusting for other confounding variables (OR 1.22, 95% CI 1.14 to 1.31).
In univariate analysis, the genotype (B1B2+B2B2) was associated with an increased risk of familial AMI. Compared with subjects with the B1B1 genotype, the OR for subjects carrying the B2 allele was 2.4 (95% CI 1.5 to 4.0). The impact of the B2 allele on familial AMI risk was also confirmed after adjustment for age, gender, smoking habits, history of diabetes, hypertension, and hyperlipidemia (OR 2.4, 95% CI 1.2 to 4.6) and was as strong as the risk determined by smoking and age. Conversely, the association between fibrinogen genotype and familial AMI was lost after adjustment for fibrinogen levels (OR 1.3, 95% CI 0.7 to 2.5).
Fibrinogen levels in cases and control subjects, according to the
Bcl I fibrinogen genotype, and the interaction
between case status and genotype are shown in Table 3
. After correction for gender and
smoking habits, the (B1B2+B2B2) genotype was still associated
with higher fibrinogen levels in both cases and control subjects. The
difference in fibrinogen levels between cases and control subjects was
more pronounced in subjects carrying the genotype (B1B2+B2B2)
(337±59 versus 262±58 mg/dL) than in that between cases and control
subjects with the B1B1 genotype (276±58 versus 232±59 mg/dL),
the interaction between case status and Bcl I
genotype on fibrinogen levels being significant
(P=.042). In the stepwise regression analysis
including genotype, age, sex, smoking habits, history of
diabetes, hypertension, and hyperlipidemia, the
Bcl I genotype accounted for 14% (the model
explaining 24% of the variance) and 8% (the model explaining 10% of
the variance) of the fibrinogen variance, respectively, in cases and
control subjects.
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| Discussion |
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Several studies have shown a strong association between elevated plasma fibrinogen levels and risk of ischemic heart disease.1 2 3 4 5 6 7 10 11 12 A role for increased plasma fibrinogen levels has been also described in the risk for peripheral artery disease,9 mortality associated with peripheral artery disease, and graft occlusion after femoropopliteal vein bypass. Presently, the main concern about fibrinogen as a risk factor is whether the increase in fibrinogen simply reflects the atherothrombotic disease or whether it plays a causal role in its development. Our findings offer evidence of an active role of fibrinogen in cardiovascular disease, by demonstrating that a fibrinogen genotype, associated with higher fibrinogen levels, is independently associated with AMI. In this case, there is a genetic predisposition to have high levels of fibrinogen, which in the presence of an interactive condition, such as inflammation, predisposes the subject to develop AMI.
The number of B2B2 subjects in our AMI sample was smaller than that expected from the Hardy-Weinberg equilibrium. This could be due to a particular severity of the disease in such patients that predisposes to early fatal events. However, prospective studies are required to support such a hypothesis.
We selected patients with a family history of ischemic vascular disease, because genetic variants related to thrombosis development should be found more frequently in such patients than in those without a family history. The strong correlation between the increase in the B2 allele and familial AMI suggests the possible inheritance of the disease: it is conceivable that the transmission of the B2 allele, in combination with other risk factors for AMI, may account for its development in families.
Only 30% of all ischemic cardiovascular events can be predicted on the basis of established risk factors like hypercholesterolemia, smoking, overweight, diabetes, age, gender, and hypertension.27 Although many of these factors have been related to myocardial infarction, in our study, the impact of the B2 allele on AMI risk was independent from them. On the other hand, the distribution of genetic polymorphisms can be more strongly influenced by the geographic and ethnic origin of the subjects under study than by the conventional factors related to AMI. The subjects participating in this study were all Italian and were homogeneously distributed throughout the main Italian geographic areas. Moreover, the catchment areas for cases and control subjects were comparable.
The finding that the B2 allele of the Bcl I fibrinogen polymorphism is a risk factor for familial AMI is in agreement with those of the Edinburgh Artery Study,9 which reported a higher frequency of the B2 allele in patients with peripheral artery disease than in control subjects. However, they did not find any correlation between the polymorphism and blood fibrinogen levels, either in patients or in control subjects. More recently Behague et al17 showed an association between genetic variants of the ß-fibrinogen locus and the severity of coronary artery disease in patients with AMI. However, they failed to demonstrate a clear association between the same alleles and AMI: only patients with a more severe coronary artery disease differed from control subjects in B2 allele distribution. It is therefore conceivable that this polymorphism could play a role (not yet defined) in the complex interactions between fibrinogen and atherosclerotic plaque evolution.28 29 30 We studied patients with a family history of thrombosis, which is considered by itself an independent risk factor for AMI11 24 31 32 and could overexpress the noxious effect of other risk factors with a potential genetic component such as hypertension, hyperlipidemia, or diabetes. In such a condition, the "multiple risk factor" theory33 for disease implies that certain factors interact cumulatively to create high risk individuals with a particularly severe form of the disease.
High fibrinogen levels could be the candidate factor mediating the unfavorable effect of genetic predisposition linked to B2 allele on coronary atherosclerosis and thrombosis.9 34 This supposition could be confirmed by the marked difference in mean fibrinogen levels we found between AMI patients and control subjects carrying the B1B1 genotype versus those with the (B1B2+B2B2) genotype. Subjects carrying the B2 allele showed much higher fibrinogen levels than those carrying the B1 allele, after adjustment for smoking habits. In a stepwise regression analysis, the Bcl I polymorphism accounted for 14% and 8% of the total variance of plasma fibrinogen levels, respectively, in cases and control subjects. If the B1/B2 site was related to fibrinogen levels through the response of fibrinogen to stimuli, such as inflammation, then one would expect the difference between cases and control subjects in B2 subjects to be greater than the difference between cases and control subjects in B1 subjects. This appears, indeed, to be the case. Plasma fibrinogen levels were significantly different between AMI and control subjects, a difference mainly ascribed to the (B1B2+B2B2) genotype. Indeed, a formal analysis of interaction between genotype and fibrinogen levels showed that the difference in fibrinogen levels between cases and control subjects was significantly greater in (B1B2+B2B2) subjects than in subjects with the B1B1 genotype. Moreover, the association between the fibrinogen genotype and the risk of familial AMI was lost after correction in a multivariate analysis adjusted for the fibrinogen levels. These observations suggest that, in subjects with a family history of thrombosis, the influence of the genotype on the risk of AMI is at least in part due to its effect on fibrinogen levels.
Each polypeptide chain of fibrinogen is encoded by a separate mRNA,
transcribed by three distinct, single-copy genes that cluster in about
50 kb on the distal third of the long arm of chromosome
4,4 19 20 35 where fibrinogen genes are organized
in the order of:
-
-ß. The rate-limiting step in the assembly of
plasma fibrinogen is the synthesis of the ß
chain.22 Therefore, it is conceivable that
mutations in this gene could modulate the levels of fibrinogen, thus
increasing the risk associated with high fibrinogen levels. We studied
a polymorphism of the ß-chain gene, located in the 3' flanking
region downstream of the
-chain gene. This region does not possess
well-defined properties but could contain regulatory sequences for mRNA
synthesis. On the other hand, it could be only a marker for functional
variants in the codifying regions or in the promoter that affect the
sequence or the synthesis of the protein. The correlation between the
population data reported here and their functional meaning, therefore,
warrants further investigation.
Our study adds a new piece in the complex puzzle of factors which contribute to the risk of arterial thrombotic events. The OR associated with the B2 allele is comparable to those of hypertension, diabetes, and smoking status, although smaller than history of hyperlipidemia.
In conclusion, the novelty of the present work resides in the indication that in a population such as that with familial AMI, a link was found between (1) a genetic polymorphism of the fibrinogen gene, (2) the corresponding plasma levels of fibrinogen, and (3) the risk for a clinical event (AMI), three stages which had not been related so far in the same population. This link offers evidence for a causal role of fibrinogen in AMI and the basis for evaluating possible interventions to reduce fibrinogen levels in the prevention of this disease.
| Appendix 1 |
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| Acknowledgments |
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Received January 14, 1997; accepted April 4, 1997.
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